Most hearing loss and balance disorders are due to the death of sensory hair cells located in the auditory and vestibular systems of the inner ear. In lower vertebrates these cells are able to regenerate following damage, but in mammals this capacity is either absent or extremely limited. Regeneration, like embryonic development, requires coordination between the biochemical machinery that governs cell proliferation with that which governs cell differentiation and morphogenesis of structures like the sensory epithelium of the inner ear. The long-term objective of this proposal is to understand the signal transduction pathways mediating this coordination. These pathways are likely to be good targets for future efforts to manipulate the process of regeneration. This laboratory has recently discovered that p27kip1, a cyclin-dependent kinase inhibitor that plays an important role in regulating cell proliferation during development, is a key element in establishing normal cell numbers and morphology of the organ of Corti. In mutant mice lacking the gene for p27kip1 the sensory hair cells and supporting cells are over-produced during embryogenesis bringing about morphological abnormalities. The mutant mice are severely deaf. Abnormal cell proliferation continues postnatally in the organ of Corti, consist with a role for p27kip1 in maintaining the normally quiescent state and impeding regeneration following damage. The goal of this proposal is to test the hypothesis that p27kip1 regulates cell number and organ of Corti morphogenesis during development of the inner ear, and to investigate the mechanism of p27kip1 regulation in vivo. This hypothesis will be tested in knockout and transgenic mice, by correlating p27kip1 developmental expression with the cessation of cell division and with the morphological consequences of our genetic manipulations (Aim 1). Next, we will study the restricted pattern of p27kip1 expression during development, first by correlating temporal and spatial expression of the p27kip1 mRNA and protein levels during embryogenesis and, subsequently, by using transgenic mice to test the significance of these regulatory mechanisms for organ of Corti development (Aim 2). Finally, the role of p27kip1 in regeneration will be investigated, using the p27kip1 knockout mice for a comparison of regenerative responses between mutant and wild type animals. In addition, we will compare the regulation of p27kip1 expression in mice and in chickens, which are able to undergo proliferative proliferation in response to hair cell loss (Aim 3).